Salvage of failed pedicle screw: biomechanical comparison of calcium sulfate versus polymethylmethacrylate augmentation

The purpose of this study is to compare biomechanical differences between calcium sulfate and polymethylmethacrylate (PMMA) augmented pedicle screws as salvage techniques for failed primary lumbar pedicle screw. As the study method; lumbar vertebrae were harvested from six calf and all soft tissue was dissected and the bone mineral density of each vertebra was determined. In first stage; left or right pedicle entry side was determined with the help of intersection technique and polyaxial pedicle screws were inserted to all vertebrae. All specimens were embedded in cement, each potted specimen was mounted on the base of an Instron Machine and the pull-out tests were performed in axial direction. In second stage; the specimens were divided into two groups randomly. Calcium sulfate was used in group 1, PMMA was used in group 2 for augmentation of screw holes. After the pull-out tests of the specimens were repeated same as in stage 1, the pull-out strength results of all specimens were recorded and compared with statistical analyses. The mean bone-mineral dansitometry (BMD) of all primary screws was 1.006.4±430.4 g/cm2 and no significant difference in BMD was found between two groups (p>0.05) . In the first group; the mean POS of primary screws was 2441.3±936.4 N/m2 and after calcium sullfate augmentation, mean POS was 2499.5±1425.1 N/m2. In the first group, no statistical difference was found between these POS results (p>0.05). In the second group; the mean POS of primary screws was 2876.6±926.6 N/m2 and after PMMA augmentation mean POS was 3745.5±1299.2 N/m2, and difference of these results was statistically significant (p<0.05) . And also we found that significant difference between augmentation POS results of first and second group (p<0.05). Calcium sulfate augmentation improved mean POS when compared with the primary screw, but these results had no significant difference statistically. PMMA was found to restore most strong fixation in failed lumbar pedicle screws.